A step-and-scan exposure device moves a stage holding a reticle (i.e., a mask substrate on which a semiconductor pattern has been formed) and a stage holding a wafer (i.e., a wafer on which a semiconductor pattern would be formed) in a relative fashion and scans a distance equal to a single shot while irradiating light in the form of a slit via a mask and projection lens to thereby expose a single shot (a predetermined range) on the semiconductor wafer. In this process, the size of the exposure shot is determined by the long side of the slit (light) and the relative scan distance of the reticle stage. Therefore, the exposure shot can be increased. The exposure shot is also referred as an exposure field.
In such an exposure device, it is very Important to manage focus (a focus state of the pattern on the wafer surface). In view of this fact, the state of focus on the wafer surface in the exposure device is monitored (as used herein, the term focus management is not limited to defects produced by defocusing (non-focus), but also refers to the management of variations in the focus state within a shot or on the entire wafer surface, and variations in the dose (exposure) state). Measurement of the focus state of an exposure device includes measurement of the distribution of the focus state within an exposure shot and measurement of the distribution of the focus state of the entire surface of a wafer. Hereinafter, the former shall be referred to as the image plane or image plane measurement, and the latter shall be referred to as focus monitor or focus monitor measurement. The focus state is expressed as a numerical value that represents the extent to which the focus after exposure is displaced from the best focus or a reference state of focus. A known method for measuring the focus state of an exposure device is to expose and develop a test pattern using, e.g., a dedicated mask substrate, and measure the focus offset distance from the positional displacement of the resulting test pattern.
However, when the focus state of an exposure device is to be measured using such a method, time is required to perform the work to produce the conditions of the parameters required for measurement, and a considerable amount of time is also spent for measurement because the measurement is essentially a point by point measurement. Also, there are limitations on types of patterns and the illumination conditions of the exposure device, and the focus state can only be measured using patterns that are different from those used in actual devices.
Furthermore, in such an exposure device, the height of the mask substrate is adjusted in accordance with the height of the wafer stage in order to match the focus of the projection lens (focusing). However, the focus cannot be matched by a mere primary adjustment of the height of the mask substrate in the case that the image plane (of the pattern) is tilted by the projection lens or the like. In view of the above, such an exposure device measures the optimal focus conditions prior to wafer exposure. A known method for determining the optimal focus conditions is to, e.g., expose and develop a pattern for measurement while the focus is varied for each area smaller than a single slit, and determine conditions that will achieve the best focus on the basis of a regular reflectance image of the resulting pattern (e.g., see patent document 1). In this case, the regular reflectance image of the pattern is magnified and observed using a microscope and imaging elements, the conditions in which the contrast between the resist pattern (line) and space is maximum are determined to be the conditions of best focus.
However, in the case that the optimal focus conditions are determined using such a method, there are cases in which the required precision cannot be satisfied in that the thickness of the resist (resist film reduction) varies due to variation in exposure energy, and pattern loss and other impacts readily occur due to excessive defocus. Control errors end up being included and precision is reduced during image plane measurement within a shot because the focus is varied and exposure is carried out for each area smaller than a single shot. The semiconductor pattern image formed by the photo resist on the wafer may also tilt in a relative manner due to errors that occur during reticle-stage or wafer-stage scanning, and compensation for such errors cannot be made.